<div class="col-sm-3 col-md-2 sidebar">
  <ul class="nav nav-sidebar">
    <li class="<?php if( @$_GET['active'] === '0' ){ echo "active"; }?>"><a href="<?= BASE_URL ?>dashboard/">Dashboard</a></li>
    <li class="<?php if( @$_GET['active'] === '1' ){ echo "active"; }?>"><a href="<?= BASE_URL ?>admin/">Administración usuarios</a></li>
    <li><a href="salir.php">Salida Segura</a></li>
  </ul>
</div>